[size=200pt]1.using any ING editor open file gta3.IMG :
E:\Rockstar Games\GTA San Andreas\models\gta3.IMG
2.try to find orginal model and txd for example:
admiral.DFF
admiral.TXD
3.delete it or rename
for example
OLD_admiral.DFF
OLD_admiral.TXD
4.add new one (your custom car mod)
admiral.DFF
admiral.TXD
5.rebuild arhive and save[/size]
thats all (in IMG)
handling doesn't matter in sa-mp